(1) Field of the Invention
This invention relates to the treatment of subterranean formations of nonuniform permeability, and more particularly concerns an improved method for selectively plugging the more permeable water flow channels of an earth formation. The method of this invention is especially useful in promoting more uniform fluid injection patterns, such as are desirable in the secondary recovery of petroleum by displacement with flood water, and in achieving water shutoff in producing wells.
(2) Description of the Prior Art
Since only a portion of the oil contained in a petroleum reservoir can be recovered by primary methods, it has become conventional practice to employ various secondary and tertiary recovery techniques to produce additional quantities of oil not economically recoverable by primary methods. Of the various secondary and tertiary recovery methods available, one of the most widely practiced techniques is the displacement of oil from the reservoir with a driving fluid such as flood water injected for that purpose. Normally, in carrying out the flooding process, a series of input wells spaced apart from one or more producing wells are drilled into and opened to the oil-producing strata. The injection well locations with reference to the production wells are selected to afford a desired flood pattern, the selected pattern depending in part upon field conditions, the location of existing wells, and the operator's preference. Aqueous drive fluid, such as water, brine, or a viscous aqueous polymer solution, is forced into the input wells under pressure, and out into the surrounding oil-bearing strata towards the producing well or wells. While waterflooding has been rather widely practiced in recent years, it is not without considerable operating problems and economic limitations, particularly those associated with low oil recoveries in proportion to the amount of water injected. Various surfactant and solvent floods have been proposed as means for recovering additional quantities of oil over that recoverable by conventional waterflooding. However, these processes face serious operating problems when practiced in heterogeneous formations containing strata or channels having permeabilities substantially higher than the bulk of the formation.
One of the major problems encountered in a flooding operation is breakthrough of the flooding medium from the flood front to the producing well relatively early in the displacement process, and rapidly increasing producing water/oil ratios following the initial breakthrough. These difficulties result from the displacing medium channeling or fingering through the oil-bearing structure to the producing well, thus bypassing large zones of the oil-bearing strata. The reason for the channeling of the flooding medium to the producing wells and the resulting low oil recovery is due, in part, to the peculiar structure of the oil-bearing strata. Underground oil reservoirs, in most cases, consist of layers of sand or rock and, since no reservoir rock is perfectly uniform in composition and structure, the permeability will vary across the rock face or stratum. Also, fractures, cracks, vugs and other anomalies can promote channeling of the displacement fluid.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid builds up in a wide bank in front of the driving fluid which moves uniformly towards the producing well. To keep this bank of oil intact, and constantly moving towards the producing well, a substantially uniform permeability must exist throughout the strata. If this uniform permeability does not exist, or is not provided, the flooding fluid will seek the areas of high permeability, and channeling occurs with the consequent loss of some driving fluid energy and the appearance of excessive amounts of driving fluid in the producing well. Moreover, as the more permeable strata are depleted, the driving fluid has a tendency to follow channels and further increase consumption of the flooding media to the point where the process becomes uneconomical. This maximum limit, in terms of recovered oil, can be as high as 100 barrels of driving fluid per barrel of oil. It is of course desirable to operate at much lower driving fluid to oil ratios, and preferably not more than 15, and normally a maximum of 5 to 10 barrels of driving fluid per barrel of recovered oil is considered an acceptable operating condition, particularly where the driving fluid is a low cost agent such as flood water.
While a uniform flood front with reduced fingering can be obtained in some formations with a drive fluid rendered more viscous by the addition of various water-soluble polymers, or other materials capable of imparting higher viscosity to the flood water, viscous waterflooding is often ineffective in formations having severe heterogeneity as the mobility of the flood water cannot be economically reduced sufficiently to prevent or substantially restrict channeling of the flooding medium. Also, smaller volumes of aqueous polymer solution are sometimes injected into a heterogeneous reservoir to divert subsequently injected flooding media into the less permeable strata. However, such treatments are often ineffective or only partially effective in highly stratified formations.
Where a heterogeneous formation is flooded, it is usually found that the flood water or other flooding medium introduced into the injection well will enter the various strata open to the well at different flow rates depending upon the permeability of the individual strata relative to the permeabilities of the other strata in the formation. A water injection well of this type is said to exhibit a nonuniform injection profile. Heretofore, it has been considered that nonuniform injection profiles were symptomatic of channeling and the resulting poor flood water conformance. Accordingly, it was believed that selective plugging or other treatment to improve or correct the injection profile would ameliorate channeling and result in improved flood water conformance and increased oil recovery. However, most commercially practical selective plugging treatments affect the permeability of the strata only at the well face or for a few feet distance from the well. Since adjacent strata are often in fluid communication throughout all or a substantial part of the formation subjected to flooding, cross flow between the various strata exists and severe channeling develops even through a substantially uniform injection profile is obtained. Thus, in many applications, treatment of the injection wells to improve the water injection profiles has no demonstrable effect on oil recovery. Hence, need exists for an improved flooding process for use in heterogeneous formations that will minimize channeling of the flooding medium or bypassing of large areas of the formation, or for a method for improving the effectiveness of the conventional selective plugging processes to obtain these goals.
Another problem associated with the production of oil from oil-bearing formations containing highly permeable water channels communicating the production well with a water zone is the intrusion of water into the well. Not only does this water intrusion cause production and disposal problems, but more importantly the beneficial effect of the natural water drive is at least in part lost thereby adversely affecting oil recovery. It is advantageous to at least partially plug the more permeable water channels so as to render the formation more uniformly permeable and to increase the sweep efficiency of the water drive, or alternatively to shut off the water intrusion. Heretofore, selective plugging of these water channels has presented great difficulty, it being necessary to effect at least a partial water plug without adversely affecting the permeability of the formation for oil production.
Thus, many processes have been proposed for plugging high permeability and/or water-producing zones wherein there is injected into formations containing such zones a wide variety of viscous solutions and/or plug-forming solutions. However, in many instances adequate plugging of such zones has not been achieved.
In one previously used conventional method of permeability adjustment within a formation by the separate injection of reactive chemical solutions, there is sequentially injected into a formation: (1) a first aqueous chemical solution which may or may not contain a thickener, (2) an inert spacer liquid and (3) a second aqueous chemical solution which will react with the first aqueous chemical solution to form a solid precipitate upon contact therewith. In another previously used conventional method there are injected into a formation a number of cycles of material, each cycle comprising: (1) a first aqueous chemical solution, (2) an inert spacer liquid and (3) a second aqueous chemical solution which reacts with the first aqueous chemical solution to form a solid precipitate as described above. In both of the previously described methods the injected fluids move out into the formation in more or less circular bands from the injection or input well, the symmetry of the bands depending upon the uniformity of the structure. The injected solutions tend to preferentially enter the more permeable water channels. As the bands of solution move outwardly, the chemical solution bands gradually approach each other as the spacing medium band narrows due to the geometric relationship of the distance of the band from the well. At some point in the formation there is mixing of the first aqueous chemical solution and the second aqueous chemical solution. When this occurs a solid precipitate forms which at least partially plugs this zone. As the most permeable channels become partially plugged, fingering of the solutions into other somewhat less permeable zones increases so that ultimately the formation is rendered more uniformly permeable.
Thus, in all methods of permeability adjustment by the separate injection of reactive chemical solutions, it is necessary to avoid mixing of the two reactive solutions and the resulting precipitate formation until the solutions are positioned in the more permeable zones of the formations. When precipitation does occur, it is desired that an effective plug be formed. Formation of a plug in the proper location which is strong enough to be capable of withstanding the pressure of encroaching formation water or injected fluids have proven difficult. The method of the instant invention results in more effective mixing of the reactive solutions in the formation; hence, formation of an especially strong plug capable of effectively plugging the high permeability zones.
Accordingly, a principal object of this invention is to provide a method for controlling the permeability of a subterranean formation of nonuniform permeability.
Another object is to provide a method for reducing channeling in a flooding process of the flooding medium from an injection well to a producing well via a high permeability zone.
Still another object is to provide an improved method for selectively plugging water-producing zones in a subterranean formation.
A further object is to provide a method for plugging a relatively high permeability zone of a subterranean formation which method is effective regardless of the direction of flow of fluids through the formation.
Other objects, advantages and features will become apparent from the following description and appended claims.